The object of the present invention is a method for eliminating a temperature dependant dispensing error in connection with the dispensing of liquids.
Specifically the invention is used in connection with a dispensing method, wherein the dispensing takes place by means of two chambers connected to each other over a gas passage, wherein the first chamber communicates, in addition to the said gas passage, with the liquid to be dispensed, and the second chamber is gas tight except for the said gas passage, whereby, in order to take up liquid in the dispensing device, the volume of the second chamber is increased, resulting in gas flowing therein from the first chamber, and in turn, liquid to be dispensed flowing into the first chamber until a pressure equilibrium between the chambers has been reached.
The object of the invention is also a liquid dispensing device comprising two chambers interconnected over a gas passage, the first chamber communicating, in addition to the said gas passage, with the liquid to be dispensed, and the second chamber being gas tight except for the said gas passage, and containing means for regulating the volume thereof.
The method according to the invention is applied typically, but not solely, when pipetting liquids using an electronic or mechanical pipette, and it allows for the elimination of or at least a substantial reduction of a liquid dispensing error resulting from temperature differences between the sample and the pipette.
In laboratory analyses pipetting and dispensing of samples and reagents which are usually in liquid form comprises the most difficult stage in the process of obtaining an accuracy of, for example, less than half a percent in accuracy as well as in reproducibility. The pipetting accuracy using the more old-fashioned technique has usually been in the magnitude of one microliter (with an inaccuracy of even 5 to 10%, especially at smaller volumes), if not worse (Rodgerson et al, Clin. Chem. 20/1, 43-50, 1974 and Pardue et al, Clin. Chem. 20/8, 1028-1042, 1974). Using the modern techniques of today, the accuracy of both mechanical and electronic pipettes varies in the range of 0.1-5% (Suovaniemi: Dissertation, ISBN 952-90-5248-0, University of Helsinki, 1994).
The pipetting inaccuracy depends on both the apparatus and also its user. The effect of human error is more often evident especially in connection with the use of mechanical pipettes as compared to the use of electronic pipettes. The quality of the pipette tip and the manner of its use has its own small effect, except when the tip is placed on to the pipette by hand. In this case the tip may become contaminated or compressed so that the tip regains its shape during the first pipetting operations and in connection therewith draws an excess liquid into the tip. This error can be eliminated e.g. by using vertically packaged tips or by handling the tips with extreme care without changing their form when mounting them on a pipette. The use of an electronic pipette removes the human pipetting errors, as well as the use of automatic calibration of an electronic pipette in connection with each use (Hattori, Super Pipetter Seminar: Standardized Risk Factor and Support to Validation, Biohit, 1994). Improving these factors usually leads to pipetting results which are only 0.1-0.5% more accurate.